The present invention relates to a multi-blade ditching machine or a trencher which is used for burying submarine cables or the like in the sea floor and/or for pulling them above the water surface, and more particularly to an improvement in blades of a multi-blade ditching machine.
To protect submarine cables or the like from fishing tools, it has been the practice in many countries of the world to bury the cables or the like in the sea floor and to pull out such buried cables or the like by ditching a ditch for repair in case of faults. For this purpose, ditching machines which ditch the soil or sediment of the sea floor to a desired depth have been used, and such ditching machines are called cable-buriers or cable-searchers depending on the purpose thereof.
Although the structure of the cable-buriers is somewhat different from that of the cable-searchers due to the difference in their pusposes, the essential ditching portions of both the cable buriers and cable-searchers are similar to each other, and such ditching portions use water jets or plows for ditching trenches on the sea floor. Conventionally, two types of ditching portions with plows have been used, i.e., single-blade type ditching portions and multi-blade type ditching portions.
The present invention relates to a multi-blade plow type ditching machine or trencher.
In order to facilitate the easy understanding of the present invention, a prior plow type multi-blade trencher will be described in accordance with FIGS. 1 through 4. That prior art is shown in U.S. Pat. No. 4,053,998, and U.K. Pat. No. 1,477,815.
FIG. 1 shows a cable ship under the operation of laying a submarine cable. In the figure, the reference numeral 100 shows a ditching machine or a trencher, 101 is the sea bottom, 102 is the trench thus dug, 103 is the communication cable to be laid, 104 is a wire for towing a trencher by a cable ship or a work ship, 105 is a work ship, 106 is a sea surface. As the work ship 105 tows the trencher 100 through the towing wire 104, the trench 102 is provided on the seabottom, and the cable 103 is laid and buried in said trench.
FIGS. 2 and 3 show the prior ditching machine, and FIG. 4 is a schematic diagram illustrating the relation between the blade arrangement of the ditching machine in FIGS. 2 and 3. In FIGS. 2 and 3, the reference numeral 1 is the stabilizing wing, 2 is the center body, 2a is the side plate connected to said center body, 3 is a joint between the stabilizing wing and the center body, 5.sub.1 through 5.sub.n are blades for digging a trench and pushing away the ditched soil. In the embodiment shown in FIGS. 2 and 3, there are seven blades and so n=7. Also the reference numeral 6 is a towing wire (corresponding to 104 in FIG. 1), 7 is the cable inlet, 8 is the bottom of the trench thus dug, 9 is a cable to be buried, 10 is a communication repeater inserted between the cables, 13 is a rotational axis of a pushing roller, 14 is a frame of a pushing roller, 15 is a pushing roller, 18 is a second small roller, 19 is a tail roller, and 20 is a guide for the pushing roller 15.
The ditching machine or the trencher shown in FIGS. 2 and 3 is towed by the work ship by the towing wire 6, and digs the seabottom by the depth H from the level S.sub.1 to the level S.sub.2 utilizing the blades 5.sub.1 through 5.sub.n. The cable and the repeaters taken off from the work ship are laid in the trench thus dug through the center body 2 of the trencher. The stabilizing wing 1 of the trencher functions to stabilize the posture of the trencher on the seabottom, and the pushing roller 15 pushes the cable to the bottom of the trench by the weight of the roller 15 itself. So the cable is taken off through the path between the pushing roller 15 and the second small roller 18 to the trench bottom 8. The trail roller 19 is utilized when a cable is not buried but is merely put on the seabottom. When only the trail roller 19 is used, the ditching machine is lifted on the deck of the work ship and is placed in the upside-down on the deck of the work ship. The cable 9 is then taken off through said trail roller 19 to the seabottom.
FIG. 4 shows the front view of the arrangement of the blades 5.sub.1 through 5.sub.n, and for the sake of the simple explanation only the right half of the blades are shown in the drawing. Of course the left half of the blades is symmetrical with the right half. At first, the first blade 5.sub.1 ( (1)-(1) ) digs the wide ( width W.sub.1) and shallow (depth d.sub.1) trench, which is deepened by the succeeding blades 5.sub.2 through 5.sub.n. When the last blade 5.sub.7 digs the trench, said trench has the width W.sub.7 and the depth (d.sub.1 +d.sub.2 +d.sub.3 +d.sub.4 +d.sub.5 +d.sub.6 +d.sub.7). It should be appreciated in FIG. 4 that the actual digging of the trench is accomplished by the ditching portion at the center of the blades and the excavated soil is placed on the outer space of the trench thus dug by the soil pushing portion at the top of each blade.
However, the prior ditching machine shown in FIGS. 1 through 4 has the disadvantages that the excavated soil has the tendency to be dropped down again in the trench which has just been dug, because of the shape of each blade and the low pressure generated behind each blade. Then the towing tension of towing wire becomes extremely high, and increased towing power of the work ship is required.